That is an excellent question and one that is best answered by your primary care doctor
or a doctor specializing in teeth such as a dentist
. There are several causes for darkening of the teeth, some are related to what you eat, as these can stain the teeth, some are related to hygiene, and some are related to other medical problems that you may have that can cause this, or medications that you are taking that can lead to darkening of the teeth. From the limited history provided, it is impossible to know which of these causes could be the offending agent for why your teeth are darkening, and also why it is only the bottom 4 but that is why it is important that you see a doctor who can evaluate and treat you for this. One other potential cause that you didn't mention was that although you don't smoke, people who use chewing tobacco can have this type of darkening of their bottom teeth and so if this is the case, you should stop doing this at once and see a doctor to see what options to you have to fix this color change you are experiencing.